1. Field of the Invention
The present invention is directed to a system for dynamically setting the line scan time or charge transfer clock timing and the color channel integration time for a multiple color channel charge coupled device (CCD) sensor in a scanner and, more particularly, to a system that dynamically determines the integration or exposure time for the channel that requires the longest required exposure time, sets the charge transfer clock, which governs sensor line scan time, responsive to the required exposure time, sets the delay or dead time of the more sensitive channels relative to the transfer clock and adjusts the exposure time for film density.
2. Description of the Related Art
The specific function of a film scanner 10, such as represented schematically in FIG. 1, is to measure optical density for a plurality of colors at many points on film 12 being scanned. The density of each pixel, or smallest region of the film 12 that is sensed, is measured by illuminating the region with light of a known light intensity and measuring the amount of light which is transmitted through the film 12 over a specific spectral range. The film scanner 10 typically includes illumination optics 14 which projects light through the film 12, a lens 16 which focuses the transmitted light through spectral separation filters and onto an optical detector 18 and measuring electronics 20 which determines the amount of light that is sensed by the detector 18. The illumination optics 14, as illustrated in FIG. 2, include an illumination source (or lamp) 21, bandpass and balance filters 22 to condition the broad spectral characteristics of the light from the lamp 21 and an optomechanical device (integrator 24 with condensing optics 26) which spatially conditions the light to irradiate a region of the film which includes the pixels being measured with a stable, intense beam of light. The lens 16 (FIG. 1) collects the light which is transmitted by the film 12 and transports it through the spectral separation filters to the detector 18. The optical detector 18 converts light to an analog electrical signal and the measuring electronics 20 is responsible for measuring this electrical signal at the proper time, converting it to a density measurement, correcting it for instrument errors and translating it into a digital form suitable for transmission to a computer or other device. The detector of a three color scanner includes three linear arrays of sensors that are scanned across film to convert the film image into a digital image.
Film scanners are imperfect instruments which must be periodically calibrated. The calibration of a film scanner may be performed either with no film present or with film of a known density mounted in the film holding frame of the scanner. The amount of light which reaches the optical detector 18 is not only a function of the calibration film, it also will vary due to manufacturing tolerances and aging effects in the optical components of the film scanner. Also, multi-format film scanners may operate at a plurality of optical conjugates, resulting in variations in system magnifications which result in wide variations in the amount of light which is collected by the lens 16 and transmitted to the optical detector 18.
The radiometric calibration process of a film scanner consists of determining the amplitude of the analog signal (under controlled conditions) and adjusting some parameter(s) which will amplify or attenuate the signal amplitude. This process is repeated iteratively until the desired signal level is achieved. To date, several on chip exposure control methods for CCD imagers have been discussed and which control the amplitude of the output signal either by discarding unwanted charge accumulated over a percentage of the integration period (U.S. Pat. Nos. 5,105,264 and 5,233,428) or by modifying the bias voltages applied to the device electrodes (U.S. Pat. No. 4,553,168). Other schemes have been devised and discussed by which the incident light is attenuated by an electronically controlled shutter which is external to the CCD (U.S. Pat. No. 5,247,367).
A common goal of all film scanners is to achieve as high a quality digital image as is possible in the shortest amount of time. A film scanner which is capable of digitizing a wide range of film formats must be capable of a wide range of optical conjugates (or system magnifications). The system radiometric requirements for any given magnification may be calculated relative to the known systems radiometric requirements at some reference magnification, however, scanner to scanner variations and variations in a given scanner over time require that the system be recalibrated at this reference magnification quite often. It is desirable to have a method which can optimize the radiometric performance of such a scanner for any possible setup condition or state. Use of traditional exposure control methods in a multiple format film scanner is inefficient for this purpose. For example, a film scanner which utilizes the same illuminator for each film format must adjust the integration time by a factor of 4 when going from a magnification of 0.5 to 2.0. If on chip electronic exposure control or a mechanical iris or shutter is used to compensate for this difference, the magnification 0.5 setup would scan at a speed which is four times slower than the optimal speed. Also, when the above mentioned techniques, which "throw away light" are used, the system not only suffers from inefficient scan times, but also lower quality of the resulting digital image because undesirable integration time dependent artifacts in the optical detector are enhanced and/or the lens is operated at a less than the optimal f-stop.
What is needed is a system which will dynamically calibrate a scanner to optimize the line scan speed for any set up condition of the scanner.